2 - Raspberry PI Community Projects

that gets started) is known as dom0, and has a special role, since only this domain can control
the hypervisor and the execution of other domains. These other domains are known as domU. In
other words, and from a user point of view, the dom0 matches the “host” of other virtualizaton
systems, while a domU can be seen as a “guest”.
CULTURE
Xen and the various versions
of Linux
Xen was initially developed as a set of patches that lived out of the official tree,
and not integrated to the Linux kernel. At the same time, several upcoming
virtualization systems (including KVM) required some generic virtualizationrelated
functions to facilitate their integration, and the Linux kernel gained
this set of functions (known as the paravirt_ops or pv_ops interface). Since
the Xen patches were duplicating some of the functionality of this interface,
they couldn't be accepted officially.
Xensource, the company behind Xen, therefore had to port Xen to this new
framework, so that the Xen patches could be merged into the official Linux
kernel. That meant a lot of code rewrite, and although Xensource soon had
a working version based on the paravirt_ops interface, the patches were only
progressively merged into the official kernel. The merge was completed in
Linux 3.0.
➨ http://wiki.xensource.com/xenwiki/XenParavirtOps
Although Squeeze is based on version 2.6.32 of the Linux kernel, a version
including the Xen patches from Xensource is also available in the linuximage-2.6-xen-686
and linux-image-2.6-xen-amd64 packages. This distributionspecific
patching means that the available featureset depends on the distribution;
discrepancies in the versions of the code, or even integration of code
still under development into some distributions also mean differences in the
supported features. This problem should be greatly reduced now that Xen has
been officially merged into Linux.
➨ http://wiki.xen.org/xenwiki/XenKernelFeatures
Using Xen under Debian requires three components:
NOTE
Architectures compatible
with Xen
Xen is currently only available for the i386 and amd64 architectures. Moreover,
it uses processor instructions that haven't always been provided in all
i386-class computers. Note that most of the Pentium-class (or beer) processors
made aer 2001 will work, so this restriction won't apply to very many
situations.
CULTURE
Xen and non-Linux kernels
Xen requires modifications to all the operating systems one wants to run on
it; not all kernels have the same level of maturity in this regard. Many are
fully-functional, both as dom0 and domU: Linux 2.6 (as patched by Debian)
and 3.0, NetBSD 4.0 and later, and OpenSolaris. Others, such as OpenBSD
4.0, FreeBSD 8 and Plan 9, only work as a domU.
However, if Xen can rely on the hardware functions dedicated to virtualization
(which are only present in more recent processors), even non-modified
operating systems can run as domU (including Windows).
Chapter 12 — Advanced Administration
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